A hydrogel is a gel which contains water by hydration. Such hydrogels have been known for years but recently as the interest in functional materials has increased, unique properties of hydrogels have attracted special interest. Hydrogels generally are sparingly irritating to the tissues and are superior in permeability to various substances. Hydrogels also show improved antithrombotic activity with increasing water contents. Hydrogels are, thus, considered to be very promising medical raw materials.
Those skilled in the art know high polymers capable of forming such hydrogels such as gelatin, carrageenin, alginic acid, 2-hydroxyethyl polymethacrylate, polyacrylamide, polyvinyl alcohol and so on. Hydrogels can be produced by a number of methods. For polyvinyl alcohol (hereinafter referred to sometimes as PVA) hydrogel as an example, when a concentrated aqueous solution of PVA is prepared and allowed to stand at a temperature below room temperature, it progressively gains viscosity and ultimately gives a gel.
However, the PVA gel prepared in this manner is not satisfactory in mechanical strength. Therefore, a method was proposed which comprises freezing a concentrated aqueous solution of PVA at a low temperature in a brief time and thawing it at room temperature(Japanese Patent Application Kokai No. 52296/1975). A method has also been proposed in which a crosslinked gel is produced using a crosslinking agent such as formaldehyde or by irradiation with .gamma.-rays, for instance. The resulting hydrogels have definite mechanical strength and flexibility and can be used as medical materials such as vehicles for slow-release of drugs, carriers for immobilization of enzymes and microbial cells, thermal carriers for cold retention, and bases for controlled release of aromas and perfumes.
While the hydrogel generally has the above-mentioned beneficial properties, the gel tends to shrink with time as it releases the entrapped water. Therefore, the hydrogel has not only poor formulation stability but, when such a hydrogel preparation is applied to the skin, it elicits irritable responses owing to vaporization of entrapped water.
The present inventors have conducted an intensive investigation to overcome these disadvantages. As the result, it has been found that a hydrogel with a minimum of change in water content and an improved formulation stability can be obtained by incorporating a water-containing component capable of containing a large amount of water into a hydrogel. The present invention is predicated on this finding.